The invention relates to image pickup devices, particularly devices for picking up infrared images of scenes and converting these images to electrical signals. The electrical signals may then be converted into visible images of the infrared scenes.
A known image pick up device is shown in FIG. 1. In this image pickup device, light rays 8 from a distant object 10 are converged by a double convex lens 12. The converging light rays are reflected by a scan mirror 14 to produce an image 16 of the object 10 on a detector array 18. The detector array 18 is, for example, a linear array extending perpendicular to the plane of the drawing.
In order to scan the image 16 across the detector array 18, the scan mirror 14 is pivoted around axis 20. Because the scan mirror 14 is reflecting converging rays 8 (as opposed to the parallel rays between the object 10 and the lens 12), this image pick up device is sometimes referred to as a convergent beam scanner.
A number of problems arise when one attempts to scan the image 16 across the detector array 18 in a television-compatible manner. The scan frequency for television-compatible scanning is 60 cycles per second. Each scan cycle consists of an active portion and an inactive or flyback portion. In the active portion, the pivot or tilt angle of the mirror 14 varies linearly with time. An electrical signal representing the image is produced at the output of the detector array.
During the inactive or flyback portion of the scan cycle, the mirror 14 is brought back to its initial position in order to ready it for the next active scan. During flyback, no image signal is produced.
The flyback of mirror 14 must be achieved in 8% of the 1/60 second scan cycle (0.00133 seconds). In order to accomplish such a fast flyback, the size of mirror 14 must be reduced.
However, as the size of mirror 14 is reduced, the mirror must be moved closer to the focal point 22 of lens 12. By moving the mirror closer to the focal point 22, the focusing error due to scanning a convergent beam is increased. As a result, a blurred image is scanned across the detector array.
The pickup device shown in FIG. 1 also suffers from the problem of vibration. The vibration arises from repeatedly changing the direction of rotation of the scan mirror 14.